In recent years, lithium secondary batteries have been widely used as driving power supplies for small electronic devices and the like. Such lithium secondary batteries are mainly constituted of a positive electrode comprised of a lithium compound oxide, a negative electrode comprised of a carbon material or a lithium metal, and a nonaqueous electrolytic solution. As the nonaqueous electrolytic solution, carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) are used.
A lithium secondary battery using, for example, LiCoO2, LiMn2O4 or LiNiO2 as a positive electrode material brings about a reduction of the battery performance, because the decomposition products inhibit the desired electrochemical reaction of the battery when part of the solvent of the nonaqueous electrolytic solution locally undergoes an oxidative decomposition during the charging. Such a reduction is considered to be attributed to an electrochemical oxidation of the solvent at the interface between the positive electrode material and the nonaqueous electrolytic solution.
Also, a lithium secondary battery using, for example, a highly crystallized carbon material, such as natural graphite or artificial graphite, as a negative electrode material brings about a reduction of the battery performance, when the solvent of the nonaqueous electrolytic solution undergoes a reductive decomposition on the surface of the negative electrode during the charging. Even in the case of EC, which is widely used as a solvent for the nonaqueous electrolytic solution, it partly undergoes reductive decomposition during repeated charging and discharging cycles, resulting in reduction of the battery performance.
Nonaqueous electrolytic solutions for improving the battery characteristics of such lithium secondary batteries are known, for example, in Patent Documents 1 to 3.
Patent Document 1 discloses a battery using a nonaqueous electrolytic solution in which trifluoromethanesulfonate salts such as Sn(CF3SO3)2 are dissolved to improve discharge property after the storage at high temperatures. However, this document does not describe problems of cycle property. Besides, the nonaqueous electrolytic solution in which Sn(CF3SO3)2 is dissolved is found to have a problem of unstable quality, for example due to deposits formed in the electrolytic solution during preservation for prolonged periods.
Patent Document 2 discloses a nonaqueous electrolytic solution containing a specific tin salt. This document describes, for example, a battery using an electrolytic solution containing Sn(CF3SO3)2, which exhibits improved charge and discharge efficiency at the initial stage, but does not describe a detailed mechanism regarding improvements in cycle property and storage property.
Patent Document 3 discloses a nonaqueous electrolytic solution containing a specific organotin compound or a specific organogermanium compound. This document describes, for example, an electrolytic solution containing dibutyltin (1-allyloxymethyl)ethylene glycolate or dibutyltin bis(acetylacetonate) exhibiting improved cycle property after charging and discharging cycles at a charged voltage of 4.1 V. Unfortunately, electrolytic solutions containing these organotin compounds do not significantly improve cycle property at charging and discharging cycles up to 4.2 V, and lead to a significant reduction in electrical capacity during storage in the charged state of 4.2 V.
As described above, a nonaqueous electrolytic solution containing a conventional organotin compound can improve battery characteristics to some extent but is still far from satisfaction. Nonaqueous electrolytic solutions and lithium secondary batteries with further improved long-term cycle property and storage property are needed.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2-37668
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2000-294274
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2003-173816